Can't access datacontext in multivalueconverter

Question

I have a usercontrol which I need to set a specific DataContext on. The usercontrol uses multivalueconverters. However, the binding in multivalueconverter fails to use the datacontext. It works fine for regular value converters. Anyone know what is going on? To clarify, below code is not similar to my real code, it's just a sample reproducing my issue. FOr example, my multivalueconverters take more than one argument.

XAML:

<Window x:Class="MultConvTest.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:MultConvTest="clr-namespace:MultConvTest"
    Title="Window1" Height="300" Width="300">

    <Window.Resources>
        <MultConvTest:MyValueConverter x:Key="MyValueConverter" />
        <MultConvTest:MyMultiConverter x:Key="MyMultiConverter" />
    </Window.Resources>

    <StackPanel>

        <!--This works, output is Formatted: 7-->
        <TextBlock 
            DataContext="{Binding Path=Data}"
            Text="{Binding Path=Length, Converter={StaticResource MyValueConverter}}" />

        <!--This works, output is Formatted: 7-->
        <TextBlock>
            <TextBlock.Text>
                <MultiBinding Converter="{StaticResource MyMultiConverter}">
                    <Binding Path="Data.Length" />
                </MultiBinding>
            </TextBlock.Text>
        </TextBlock>

        <!--This fails, the converter never access the DataContext of the textbox-->
        <TextBlock 
            DataContext="{Binding Path=Data}">
            <TextBlock.Text>
                <MultiBinding Converter="{StaticResource MyMultiConverter}">
                    <Binding Path="Length" />
                </MultiBinding>
            </TextBlock.Text>
        </TextBlock>

    </StackPanel>

</Window>

Code behind:

using System;
using System.Collections.Generic;
using System.Windows;
using System.Windows.Data;

namespace MultConvTest
{
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();

            DataContext = this;
        }

        public string Data
        {
            get { return "My Text"; }
        }
    }

    public class MyValueConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            int number = (int)value;
            return string.Format("Formatted: {0}", number);
        }

        public object ConvertBack(object value, Type targetTypes, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

    public class MyMultiConverter : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            // This will fail when DataContext is set. values[0] is of type Ms.Internal.NamedObject 
            // with value DependencyProperty.UnsetValue.
            int number = (int)values[0];
            return string.Format("Formatted: {0}", number);
        }

        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}

score 4 · Accepted Answer · answered Sep 13 '10 at 23:33

4

You are not going to like this ;-)

Simply put the line DataContext=this before InitializeComponent()

It seems the MultiConverter for the third textbox is triggered before the global DataContext is set, thus in effect producing a null reference error...

answered Sep 13 '10 at 23:33

Dabblernl

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That fixed it, thanks. I think this is a WPF bug with how WPF uses multivalueconverters. – Wallstreet Programmer Sep 14 '10 at 12:50
That's good to know, thank you for the input! What happens when the DataContext is set to an instance of another Class ? Will the Converter be sought after in that Class' instance ? Still getting NullReference. – Dr1Ku Dec 01 '10 at 14:36
@Dr1Ku I am not sure what your question is: you still get a NullReference exception when you use `DataContext=new MyContext()` before `InitializeComponent()` ? The Converter is a seperate class that needs the data from the Datacontext as the parameter(s) for its Convert method. – Dabblernl Dec 03 '10 at 19:14

Can't access datacontext in multivalueconverter

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